AB0125 SPECIFIC AUTOANTIBODY CONTENT OF CIRCULATING IMMUNE COMPLEXES IN SLE – PHENOTYPIC CHARACTERIZATION AND CLINICAL ASSOCIATIONS
Authors: E. Fuzzi, A. Svanqvist, C. Westerberg, A. Zickert, I. Gunnarsson, J. Rönnelid, E. Svenungsson
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Systemic Lupus Erythematosus (SLE) is characterised by autoantibody production and immune complex (IC) formation. The relative abundance of different autoantibodies within circulating ICs as compared to serum in SLE is hitherto unclear; moreover, the clinical relevance of the IC-carried fraction of SLE-specific and associated autoantibodies is mostly unknown. <h3>Objectives</h3> To investigate cross-sectionally the IC-derived fraction of SLE specific and associated antibodies, together with the total circulating C1q-binding immune complex (CIC) levels, in a large well-characterized SLE cohort. <h3>Methods</h3> We studied n=530 consecutive SLE patients (≥ 4 ACR 1982 revised criteria for SLE) who received care at a tertiary referral center for SLE. All participants gave written informed consent for inclusion. The control population consisted of region of residence, age and sex-matched controls (n=192) and n=200 local experimental controls. ACR criteria (at study inclusion), SLEDAI scores, clinical and laboratory variables were obtained at inclusion and through medical file review. At inclusion, fasting blood was drawn from an antecubital vein and was processed to serum, which was stored at -80°C. To obtain IC-derived antibodies, a novel validated method, described in detail elsewhere<sup>[1]</sup>, was employed. Briefly, sera were incubated with C1q-coated beads; the C1q-bound ICs were then eluted through two sequential washes with an acidic and alkaline solution respectively, yielding disassembled IC eluates with conserved antibody specificity. Antibody levels of Anti-dsDNA, Anti-Histone, Anti-Sm, Anti-U1RNP, Anti-Ro52, Anti-Ro60, Anti-SSB were measured in IC eluates and in serum with a bead-based multiplex assay. CIC levels were measured with a commercial ELISA according to the manufacturer's instructions. Parallel analyses of autoantibody levels in serum and in IC eluates were performed on the same day. The results were reported on a standard curve; the 98th percentile of the standard curve values of control sera were set as the threshold for positivity for binary analyses. <h3>Results</h3> CIC levels were higher and more widely dispersed in SLE patients than controls (p&lt;0.0001). A higher CIC load was associated with nephritis, arthritis and SLEDAI &gt; 6 (p&lt;0.0001). Among autoantibody specificities, Anti-dsDNA in serum was independently associated with a high CIC load (OR 4.8, 95% CI 2.5-9.5, p&lt;0.0001, see also Figure 1). Concordance between serum and IC-eluate positivity varied strongly among autoantibody specificities, being highest for Anti-SSA/SSB, and lowest for Anti-dsDNA (Table 1). Among serum Anti-dsDNA+ patients, carriers of Anti-dsDNA from ICs displayed higher Anti-dsDNA titers, lower complement levels, and higher SLEDAI scores (p&lt;0.01 for all). <h3>Conclusion</h3> Different autoantibodies are differentially represented in circulating ICs in SLE. Anti-dsDNA in serum is an independent marker of a higher CIC load, carrying facets of a more IC-mediated disease. Patients with elevated Anti-dsDNA in IC eluates, compared to anti-dsDNA only in serum, are a more clinically and biologically active subset, showing promise of IC-derived Anti-dsDNA as a potential SLE biomarker. <h3>Reference</h3> [1] Sohrabian A, <i>et al.</i> Number of individual ACPA reactivities in synovial fluid immune complexes, but not serum anti-CCP2 levels, associate with inflammation and joint destruction in rheumatoid arthritis. Ann Rheum Dis. 2018 Sep;77(9):1345-1353 <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> Enrico Fuzzi: None declared, Anna Svanqvist: None declared, Christine Westerberg: None declared, Agneta Zickert: None declared, Iva Gunnarsson: None declared, Johan Rönnelid: None declared, Elisabet Svenungsson Shareholder of: Astrazeneca and Pfizer, Speakers bureau: Janssen, Grant/research support from: Grant support from Merck.